This invention relates to resilient playground or floor (safety) mats, as embodied in the applicants' products most of which are made up of arrays of square tiles, 0.5 meter along an edge, made of a resilient material such as polyvinyl chloride (PVC) cast in moulds including perforations and surface detailing, so that the tiles have improved grip and resilience properties. Such PVC mats may include plasticisers, fillers, and colouring material and may include conductive material. Mats are intended to lie flat on a substrate and be attached by their edges to neighbouring tiles. Interlocking complementary lugs and pockets are of assistance. It has been found that more extreme environments such as outdoors in North America have been causing unexpected expansion/contraction of playground mats as a result of temperature changes. In addition a loss of dimension over a period of years is known to occur because of the gradual loss of the significant amount of plasticiser incorporated with the plasticiser. (Typically, about 20% by weight of flexible PVC is a plasticiser such as dioctyl phthalate. This substance can diffuse through the mass of the PVC). As a result, the glued joints become infiltrated with plasticiser. The inventors have realised that glued joints can be expected to fail when the plasticiser migrates into the glue itself, over time, and causes weakening of the joint. As a result, tiles lift off the substrate and/or separate from each other. Of course, the invention is in no way limited to use with the applicant's tiles; the above problem has merely stimulated the research that resulted in this invention. Many other articles constructed from plastics are amenable to a welding treatment according to the invention, as will shortly be described.